Britain is trying to build on traditionally strong ties with conservative, oil-wealthy Gulf Arab monarchies before its planned departure from the European Union.

In a joint communique, the two sides said they intended to build on trade between Britain and the Gulf Cooperation Council (GCC) states, which stood at 30 billion pounds in 2015.

"We will make it a priority, when the UK leaves the European Union, to build the closest possible commercial and economic relationship," the statement said.

This included working to remove barriers to trade and investment.

The joint communique also said the GCC states and Britain would work together to counter what they called "Iran's destabilizing activities".

Gulf Arab states say Iran is trying to expand its influence in Arab countries, including Syria and Yemen. Since March last year, Saudi Arabia and other Arab allies have been fighting a war against Iran-aligned Houthi forces that had seized control of Yemen.

The two sides also pledged to enhance defence cooperation, including efforts to defeat Islamic State and in maritime and cyber security through their new Strategic Partnership.

The statement stressed that regional conflicts can only be resolved through diplomacy.

Prior to the visit, Britain's close relationship with the Gulf States drew concern from human rights groups, including criticism of Saudi air strikes in Yemen which have killed hundreds of civilians. British military personnel have been involved with the Saudi forces.

Human Rights Watch and other organizations sent a joint letter to May urging her not ignore rights issues in pursuit of lucrative business deals. They also highlighted a political crackdown in Bahrain, where Britain has recently opened a naval base.

Asked by a reporter aboard the British warship Ocean in Bahrain on Tuesday whether Britain was selling its principles for profits by engaging politically with Gulf Arab states and selling them weapons, May said: "No. What is important is that we are able to raise these human rights issues with Gulf States and with other states around the world."

"In order to be able to do that we need to engage with those states," May said.

(Reporting by Elizabeth Piper in London and Sami Aboudi and Noah Browning in Dubai, Editing by Angus MacSwan)